1. Field of the Invention
The present invention relates generally to carbon molecular sieve (CMS) membranes, and more particularly to CMS membranes formed by stabilizing the precursors before they experience pyrolysis to provide improved permeance and selectivity equivalent to or higher than the precursor.
2. Description of the Related Art
Carbon molecular sieve membranes have shown great potential for carbon dioxide (CO2) removal from natural gas streams. In gas separation or membrane applications, a carbon molecular sieve can include a sieve that is comprised of at least ninety percent (90%) atomic weight carbon, with the remainder as various other components. CMS membranes can be formed from the thermal pyrolysis of polymer precursors.
The performance of polymer membranes can be tailored somewhat; however, the separation performance of these polymeric membrane materials has stagnated at a so-called “polymer upper bound trade-off line” related to CO2 permeability and CO2/CH4 selectivity. This trade-off can result in undesirably high methane loss along with the CO2 in the permeate stream.
CO2 permeability is a convenient measure of productivity equal to the flux of CO2, which has been normalized by the thickness of the dense selective layer and the CO2 partial pressure difference acting across this layer. The units of permeability are usually reported in “Barrers”, where 1 Barrer=10−10 [cc(STP)cm]/[cm2·sec·cmHg]. The membrane selectivity is ideally independent of the thickness of the dense layer, and equals the ratio of the permeability of CO2 to CH4 for desirable cases where the ratio of upstream to downstream total pressure is much greater than the permeability ratio of CO2 to CH4.
CMS membranes possess the ability to cross over the upper bound for dense film configurations. It is possible, using conventional CMS dense film membranes, to have CO2 permeabilities vs methane permeabilities as high as ˜75 for pure gas at 50 psia upstream and at 35° C. Some CMS membranes in hollow fiber configuration can separate CO2 from 50% CO2 mixed gas methane stream with selectivities of ˜90 for upstream pressures up to 1168 psia and at 35° C.
Though CMS hollow fiber membranes show encouraging selectivities, they show lower productivity after pyrolysis than would be expected based on the productivity increase in corresponding dense films before and after pyrolysis of the same precursor polymer. The unit of productivity for an asymmetric membrane does not contain a thickness normalizing factor, so the flux is only normalized by dividing by the partial pressure difference acting between the upstream and downstream across the membrane: 1 GPU=10−6 cc(STP)/[cm2·sec·cmHg].
There are several parameters that can influence the performance of CMS membranes, including, but not limited to: (i) the polymer precursor used; (ii) precursor pre-treatment before pyrolysis; (iii) the pyrolysis process, e.g. final heating temperature or pyrolysis atmosphere; and (iv) post-treatment of CMS membranes after pyrolysis.
Detailed investigations have been performed on CMS dense film membranes using conventional polyimide precursors such as, by way of example and not limitation, Matrimid® 5218 and 6FDA:BPDA-DAM. The chemical structures for both the polyimide precursors are illustrated in FIG. 1a for thermoplastic polyimide based on a specialty diamine, 5(6)-amino-1-(4′ aminophenyl)-1,3,-trimethylindane (Matrimid®) and 1b for 2,4,6-Trimethyl-1,3-phenylene diamine (DAM), 3,3,4,4-biphenyl tetracarboxylic dianhydride (BPDA), and 5,5-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzofurandione (6FDA) commonly referred as 6FDA:BPDA-DAM. Matrimid® 5218 is a soluble thermoplastic polyimide fully imidized during manufacturing, eliminating the need for high temperature processing, and is soluble in a variety of common solvents.
It has been shown that by tuning the pyrolysis process parameters, e.g., final heating temperature, it is possible to modify the resulting CMS membrane performance and achieve greater performance than both these precursors. Other studies have looked at the effect of pyrolysis environments on CMS dense film membranes and related membrane separation performance with different atmospheres containing varied levels of oxygen. The studies introduced the concept of “oxygen doping” on CMS membranes during the pyrolysis, as shown by way of example in US Patent Publication No. 2011/0100211 A1, the contents of which are hereby incorporated by reference.
U.S. Pat. No. 6,565,631 to Koros et al. (Koros), the contents of which are hereby incorporated by reference, extended the CMS dense film to industrially relevant hollow fiber configurations. Koros showed the synthesis of these membranes and evaluated their performance under high feed pressures and impurities. The membranes as taught by Koros are shown to be resistant under extreme conditions without significant degradation in performance. The membranes of Koros, CMS hollow fibers using 6FDA:BPDA-DAM precursors, showed CO2 permeance of ˜30 Gas Permeation Unit (GPU) with selectivities of 55 for CO2/CH4 upstream pressures up to 1000 psia and at 35° C. from mixed gas methane stream containing 10% CO2. Under the same testing conditions for Matrimid® precursor based CMS membranes, the membranes of Koros saw higher selectivities ˜85 for upstream pressures up to 200 psia and at 35° C. but with some decreased permeance of ˜12 GPU.
Low permeances are concerns for the industrial use of CMS hollow fiber membranes. Researchers in this area have tried to address this problem by relating it to the substructure morphology collapse, as shown in FIGS. 2a and 2b for a Matrimid®-based precursor. For the purposes of the present invention, we define substructure collapse as shown in the Equation
            (                        thickness          ⁢                                          ⁢                      (                          CMS              ⁢                                                          ⁢              fiber              ⁢                                                          ⁢              wall                        )                                    thickness          ⁢                                          ⁢                      (                          precursor              ⁢                                                          ⁢              fiber              ⁢                                                          ⁢              wall                        )                              )        <          0.8      *              (                              thickness            ⁢                                                  ⁢                          (                              CMS                ⁢                                                                  ⁢                dense                ⁢                                                                  ⁢                film                            )                                            thickness            ⁢                                                  ⁢                          (                              precursor                ⁢                                                                  ⁢                dense                ⁢                                                                  ⁢                film                            )                                      )              ,to be the situation in which the thickness ratio for the fiber wall after and before pyrolysis is less than 0.8 of the ratio of the thickness for a dense film after and before pyrolysis. Even for robust higher glass transition temperatures (Tg) polymer precursors such as 6FDA:BPDA-DAM, the sub-structure collapse is observed upon pyrolysis but to a lesser extent in comparison to Matrimid® precursors, shown in FIGS. 3a and 3b. 
The intensive heat-treatment during pyrolysis (above Tg) relaxes the polymer chains, causing their segments to move closer to each other, increasing the actual membrane separation thickness in asymmetric CMS hollow fibers. This increased separation thickness is believed to be the primary cause for the major permeance drop, which is defined as permeability/actual separation thickness. Although CMS dense film membrane permeability is high, due to the morphology collapse during pyrolysis, a conventional CMS hollow fiber membrane experiences a permeance drop because of increased effective membrane thickness.
Asymmetric hollow fiber membranes comprise an ultra-thin dense skin layer supported by a porous substructure. Asymmetric hollow fiber membranes can be formed via a dry-jet/wet quench spinning process illustrated in FIG. 4a. The polymer solution used for spinning is referred to as “dope”. Dope composition can be described in terms of a ternary phase diagram as shown in FIG. 4b. 
Polymer molecular weight and concentration are closely correlated to viscosity and the mass transfer coefficient of the dope which affects the overall morphology of hollow fibers. The ratio of solvents to that of non-solvents should be adjusted in order to keep the dope in the 1-phase region close to the binodal. The amount of volatile component in the dope is a key factor for successful skin layer formation.
The dense skin layer is formed by evaporation of volatile solvents which drives the dope composition toward the vitrified region (indicated by dashed line indicated by the “Skin Layer Formation” arrow in FIG. 4b). The porous substructure is formed when the dope phase separates in the quench bath and enters into a 2-phase region (indicated by dashed line indicated by the “Substructure Formation” arrow in FIG. 4b).
In this way, a desirable asymmetric morphology comprising a dense selective skin layer with a porous support structure is formed. In the process of FIG. 4a, the dope and bore fluid are coextruded through a spinneret into an air gap (“dry-jet”), where a dense skin layer is formed and then immersed into an aqueous quench bath (“wet-quench”), where the dope phase separates to form a porous substructure and can support the dense skin layer. After phase separation in the quench bath, vitrified fibers are collected by a take-up drum and kept for solvent exchange. The solvent exchange technique can play a critical role in maintaining the pores formed in the asymmetric hollow fiber
Thus, during the process of fiber spinning, as shown in FIG. 4a, sub-structure pores are formed by the exchange of solvent molecules in a dope solution with non-solvent water molecules in a quench bath during a phase separation process of the polymer from the dope solution. The pores formed do not allow a uniform well-packed distribution of polymer chains for the asymmetric hollow fiber morphology. Hence, this expanded distribution of polymer chains in the precursor fiber can be considered as a thermodynamically unstable state, promoting the tendency for the sub-structure morphology collapse in CMS hollow fibers when sufficient segmental mobility exists before pyrolysis is complete. In this case, during pyrolysis, the porous morphology of the precursor fiber turns into a thick dense collapsed layer. This change in the membrane morphology is seen to start at the glass transition temperature (Tg) of the polymer precursor. Under heat treatment above Tg, the un-oriented polymer chains enter into a soft and viscous zone which increases the chains mobility enabling them to move closer to each other. This heat treatment increases the chain packing density, resulting in the sub-structure collapse. The relaxation of the polymer precursor chains under the strong heat treatment is a primary cause for pore collapse.
Studies on the mechanism of sub-structure collapse at Tg for CMS fibers, such as Matrimid® CMS hollow fibers, and some methods to try to compensate for the membrane collapse issue have been performed in the past. For example, in order to test the hypothesis of sub-structure collapse at Tg, permeance and SEM characterization were performed shown in FIGS. 5a and 5b. The asymmetric Matrimid® precursor fiber was heated up to 320° C. (Tg of Matrimid ˜315° C.) with 10 minutes of thermal soak time under vacuum atmosphere (˜1 mtorr). The same fiber after heat-treatment at Tg is pyrolyzed using the standard pyrolysis temperature protocol, FIG. 5a, under the same vacuum atmosphere.
FIG. 5b illustrates the permeance drop experienced in CMS asymmetric hollow fiber membranes due to the sub-structure collapse occurring at Tg, tested at 100 psia and 35° C. As shown in FIG. 5b, the CO2 permeance of Matrimid® fiber heat-treated at Tg (solid square) suffers a significant permeance drop when compared to the precursor (solid diamond) and CMS hollow fiber permeance (solid triangle). Even a short soak time of ˜10 minutes at Tg is sufficient for the permeance to drop down to the maximum possible extent (0.13 GPU), which is essentially equivalent to the thickness normalized precursor dense film productivity (0.2 GPU—solid point). Because of the permeance drop, the advantage of having a high transport flux in an asymmetric precursor fiber is lost significantly or completely and the fiber can be treated as a precursor dense film with similar thickness.
The significant permeance drop of the precursor fiber at Tg indicates that the morphology of CMS fiber is essentially completely collapsed at Tg. The increase in CMS permeance (solid triangle) over the collapsed fiber is due to decomposition of volatile compounds during pyrolysis. For the collapse of CMS fibers, an important temperature zone is between the glass transition Tg and decomposition Td. Once the temperature crosses Tg and enters the rubbery region the amorphous rubbery polymer can flow, but the sieving structure does not form until the polymer begins to decompose. Therefore, minimizing the time the CMS fibers experience temperatures between these zones without introducing defects usually provides the best way to prevent or reduce permeance loss while maintaining good separation ability. But, in practice it is observed that heating at extremely fast rates leads to the creation of defects, which reduces the separation ability. Therefore, an optimum heating rate must be determined experimentally.
FIG. 6a is a SEM image of Matrimid® fiber after heat treatment at Tg, depicting the collapse morphology observed in the final CMS fibers obtained from the same precursor fiber morphology as shown in FIG. 6b. 
Conventional techniques that have been attempted to reduce or eliminate substructure collapse for polymer precursors, such as Matrimid® precursor, include, but are not limited to: puffing the porous support of polymer precursor with “puffing agents”; thermally stabilizing the fiber below the glass transition temperature Tg; and crosslinking the polymer chain in order to avoid densification.
Possible “puffing agents” are species which can decompose into large volatile byproducts upon heating and leave void volume in the carbon after decomposition. One such puffing technique includes the use of polyethylene glycol (PEG). PEG can have an “unzipping effect” upon heating at higher temperatures. Essentially all of the PEG molecules can be seen to unzip at ceiling temperatures of ˜350° C. By puffing PEG in the pores before pyrolysis, it was attempted to prevent the collapse near the Matrimid® Tg (˜315° C.). The comparison of both the TGA curves for Matrimid® and PEG (Mol wt: 3400) is shown in FIG. 7.
An advantage of using PEG is that it is soluble in water and is readily absorbed in the pores in an economical post fiber spinning step. Nevertheless, substructure collapse was seen to still occur even after PEG puffing upon pyrolysis. Without being held to any particular theory of operation, it is believed that the reason why PEG puffing does not appreciably impact sub-structure collapse is due to the wide temperature range of collapse, e.g. from Tg˜315° C. to decomposition point ˜425° C. PEG puffing is presumably not successful in stabilizing the pores, as collapse starts before the unzipping temperature of the PEG.
Pre-pyrolysis thermal stabilization of polymer precursors has also been attempted using conventional methods in both oxidative and non-oxidative atmospheres. In preliminary work, fibers were pre-heated in a furnace at 270° C., which is below the Tg, for time duration of 48 hours for pre-stabilization. After heat treatment, pyrolysis was performed using a standard protocol. Testing showed that the temperature stabilization of the pores did not make any significant impact on collapse for Matrimid®.
Another conventional method attempted is to crosslink the polymer precursor prior to pyrolysis. For example, researchers attempting to solve other issues have in the past attempted to crosslink precursors such as Matrimid® using UV radiation and diamine cross linkers. Such conventional crosslinking techniques using diamine linkers for Matrimid® based precursors have proven to be unsuccessful. As shown in the SEM images FIGS. 8a and 8b, the collapse is still observed in resultant CMS from diamine-crosslinked Matrimid® precursor fiber. Studies have indicated the diamine crosslinking to be reversible when heated at higher temperatures.
In addition to substructure collapse problems, a second challenge to CMS scale up for commercial viability is producing a large amount of CMS in a single pyrolysis run. One option in overcoming scale up problems is to pyrolyze the polymer precursor fibers in bundles but still obtain individual CMS fibers with the same or similar separation performance as non-bundled. Using conventional techniques, when pyrolyzed in bundles, polymer precursor flow can not only cause substructure collapse, but can also cause the fibers to “stick” together. When using unmodified precursor fibers according to conventional techniques, it is often necessary to separate the fibers from touching (or sticking) to each other during pyrolysis.
Thus, there is an unmet need in the art for thermally stabilized polymer precursors and asymmetric CMS hollow fiber membranes.